Process for the high temperature reaction of hydrocarbons



United States Patent PROCESS FOR THE HIGH TEMPERATURE REACTION OFHYDROCARBONS James H. Shapleigh, Wilmington, Del., assignor to HerculesPowder Company, Wilmington, Del., a corporation of Delaware No Drawing.Application May 31, 1952 Serial No. 291,065

3 Claims. (Cl. 208-47) This invention relates to high temperaturereactions and more particularly to an improved process for the hightemperature reaction of hydrocarbons in thermal or catalytic systems.

In the decomposition, cracking, or reforming of hydrocarbons, it hasbeen realized for some time that advantages are obtained by theoperation of metal tube furnaces. However, when the operatingtemperature is increased above about 900 F., certain disadvantages ariseas a result of the increased metal tube temperatures. Thesedisadvantages, as heretofore recognized in the art, have to a largeextent outweighed the advantages and have served to retard improvementand, accordingly, the use of metal tube furnaces for high temperaturereactions.

Some of the disadvantages recognized in the art were associated withlack of knowledge of scaling properties, creep strength, and resistanceof metals to sulfur compounds; especially, in systems operated at hightemperature for long periods of time. Consequently, it was an objectivein the art to carry out hydrocarbon decomposition at low temperatures.For example, in the production of hydrogen under conditions suitable forconcurrent high conversion of CO to CO large quantities of steam wereused to obtain better low temperature, theoretical equilibriumconditions. Also, for example, in the cracking of hydrocarbons toproduce olefins and products of increased value, metal tube furnaceshave not been readily accepted for operation at high temperatures andsuch furnaces as have been used have been limited to light hydrocarbonsas raw material. Disadvantages in the nature of carbon and tar formationhave retarded acceptance. Moreover, difliculties have existed inattaining proper control of both desirable and undesirable catalyticreactions, some of these caused by the catalytic elfect arising from thesurface instability of the metal tubes.

Materials of construction for high temperature systems have been one ofthe factors retarding the progress toward such systems. The currentproblems encountered with materials of construction in systems for thecracking of hydrocarbons at high temperature are mixed problems such asstrength of the material, its resistance to scaling, its ability totransfer heat, and the like. One of the principal problems lies in theactual physical and chemical changes which take place in a particularmetal during heating, particularly where the metal is held at hightemperature for long periods of time. It is known that stainless steelsemployed in high temperature equipment absorb carbon from organicreactants under certain circumstances. Furthermore, it is known thatunder reducing conditions and particularly where substantial quantitiesof hydrogen are present, such steels decarburize. Consequently, where areaction furnace is being designed for long, substantially uninterruptedoperation in high temperature hydrocarbon cracking processes, it isdifficult if not impossible to predict the actual chemical analysis ofthe metal at any particular time. In Welding procedures'where the metalis at red heat for a period in the neighborhood of thirty minutes, itis. possible to employ stabilizing agents which will prevent chromiumimpoverishment adjacent the weld and thus be reasonably certain that theweld and adjacent metal will resist intergranular attack. Suchpredictions are not possible with cracking tubes which, in theirlifetime, will be subjected, at high temperatures to a variety ofphysical and chemical conditions.

In relatively. recent years metallurgists have identified a metallicphase which forms in austenitic steels at temperatures between about1000 F. and about 1700 F. This phase is known as sigma phase and ithasbeen definitely established that its formation is accelerated in thepresence of stabilizers such as columbium and titanium which have beenadded to prevent intergranular corrosion. It has further been assertedthat sigma formation brings about increased brittleness in the metal andsome researchers have recorded tests which show that sigma isresponsible for hot shortness in welding operations which results incracking. Consequently, the art has generally considered the substantialformation of sigma phase to be detrimental and to be avoided wherestructural strength and resistance to cracking are importantconsiderations.

A further problem involved in high temperaturehydrocarbon cracking isthat of the catalytic eifect of materials employed in a given system.Moreover, the initial catalytic effect is of minor importance incomparison with the overall catalytic effect of a material during itslife. For example, a material which is not substantially stable as tocatalytic effect throughout. its life introduces another variable into agiven process. Unless the process is continually adjusted to compensatefor this variable, the maximum yield is not attained. Some processes arenot sufficiently flexible to afiord such close control and at best suchcompensating manipulations are time consuming and expensive. Processes,in general, are subject to close control to compensate for variation inthe composition of reactants, impurities in the reactants, variation inthe catalyst activity, and the like. The substantial elimination of anyone factor necessitatingprocess manipulation for compensating purposesis a definite contribution to the art.

, Thus, this invention has for its principal object an improved processfor the high temperature reaction of hydrocarbons which process may beoperated for long periods of time without appreciable deterioration ofthe equipment and without appreciable variation in the process due tothe catalytic effect of materials of construction. A further object isto provide an improved process for the high temperature reaction ofhydrocarbons. in

which thermal or noncatalytic conditions are employed as.

well as catalytic. An additional object is a process for hightemperature hydrocarbon cracking which can be carried on continuouslyfor greater periods than is now customary due to increased resistancevto tube failure. Other objects of the invention will appear hereinafterwith reference to the specification and appended claims.

In accordance with this invention, these objects are accomplished by aprocess for the high temperature reaction of hydrocarbons in which thehydrocarbons in the vapor phase undergo reaction in the presence of astar bilizecl stainless steelv having an austenitic structure. Ingeneral, in the process of the invention, hydrocarbon and steam arepassed through a heated alloy tube which may be wholly or partiallyexternally or internally heated and 3 as columbium, molybdenum, andtitanium, and mixtures thereof.

More specifically, the invention is a process for the high temperaturereaction of hydrocarbons which comprises admixing the hydrocarbon withsteam, passing the hydrocarbon-steam mixture through at least onethin-walled, austenitic alloy cracking tube, applying heat to the tube,and maintaining a section of the tube with a metal temperature gradientbetween a low of about 900 F. and a high of between about 1500 F. andabout 2400 F., said alloy tube comprising essentially carbon, chromium,nickel, iron, and at least one material from the group consisting ofcolumbium, molybdenum, and titanium, said ingredients being present inamounts from about 0.07 to about 025% for the carbon, from about 20 toabout 30% for the chromium, from about 12 to about 25% for the nickel,from about 4 to about 16 times the carbon content for the columbium,from about 2 to about 6 times the carbon content for the molybdenum,from about 3 to about 7 times the carbon content for the titanium, andthe remainder substantially iron; said alloy tube containing asubstantial amount of sigma phase when main tained at temperaturesbetween about 1000 F. and about 1700 F. The temperature gradient in thetube may be increasing or decreasing depending on the particularreaction conditions desired. Furthermore, the temperature gradient mayextend throughout substantially the length of the cracking tube. In thezone of principal reaction, metal temperatures of from about 1500" F. toabout 2400 F. are maintained. The exact metal temperatures at particularpoints are regulated in relation to the other conditions of treatment togive the desired temperature and temperature gradient within thereaction zone. Catalysts may be employed as desired, depending on thenature of the cracking reaction.

In a preferred embodiment of the invention the hydrocarbon isintroduced, in accordance with the process disclosed in my U.S. Patent2,525,276, into the alloy cracking tube or tubes along an inner pathwhich is normally a concentrically disposed injection tube. superheatedsteam is introduced into the tube and led along an outer path concentricwith the inner path. The steam in the outer path is heated indirectly bythe externally heated cracking tube and is normally at a temperature offrom 1000 F. to 1600" F. at injection level. Mixing of the two streamsand flash heating of the hydrocarbon to substantially crackingtemperature is obtained at the junction of the inner and outer paths orinjection level. Tube wall temperatures at this level are generally inthe range of about 1500 F. to about 1900 F. The resulting mixture isimmediately passed through the zone of principal reaction and crackingis obtained without appreciable carbon deposition. In this embodimentthe temperature of the hydrocarbon in the inner path is kept below thatat which carbon deposition will occur. The dilution effected by thesecondary steam flowing from the outer path brings about a desirablelowering of partial pressure of the hydrocarbon where that hydrocarbonfeed is an oil and substantially completes vaporization at injectionlevel. The highly superheated steam and indirect heating fi'om thecracking tube then effects substantially instantaneous flash heatingthrough the danger zone for carbonization, without any appreciablecarbon deposition.

The hydrocarbon feed in the process of the invention may be gaseous ornormally liquid. The liquid stock is finely diffused prior to entry intothe cracking tube, preferably by atomization by means of steam.Depending on the nature of the liquid hydrocarbon,-it may be desirablethat partial vaporization also be effected prior to entry into thecracking tube. 1

It will be appreciated that the alloy employed in the invention maycontain additional ingredients usually found in alloys of this class,such as manganese, silicon, sulfur and phosphorus. However, the alloynormally 4 will not have more than about 2% manganese, 1.5% silicon,0.05% sulfur and phosphorus, present with the essential ingredientsheretofore set forth.

Examples of the operation of the process of this invention are given inthe following in which the steel numbers designated are American Ironand Steel Institute type numbers.

Example 1 In the production of olefins, a domestic fuel oil and steam ina weight proportion of about 1 to 1 were passed through a cracking tube8 inches in diameter and 30 feet long. The cracking tube was disposed,substantially throughout its length, in a furnace aifording externalheat to the tube. The tube was heated to a temperature of about 1300 F.in the region just below the furnace arch to a maximum of about 1800 F.in the lower portion of the tube. The temperature of the exit gases wasabout 7 1250 F. The tube was composed of type 310 stainless steelcontaining 0.08% carbon and stabilized with 1.12% columbium. Nodetrimental carbon was formed and an ethylene-propylene weight ratio of0.67 to 1 was obtained.

With reference to the above example, the attainment of anethylene-propylene ratio of 0.67 to 1 represents a marked improvement inthe art and the results were consistent over a prolonged period ofoperation. Ordinarily, an ethylene-propylene ratio of about 3 to l isconsidered reasonably good process performance even where high propyleneyield is desired. The ability of the process of this invention toefliciently operate at high temperatures makes it possible to obtain lowethylene-propylene ratios, although the process is readily adaptable toobtain high ethylene-propylene ratios where high ethylene yield isdesired.

Example 2 In the production of hydrogen, a crude oil containing 2.5%sulfur and steam in a weight proportion of about 1 to 5 were passedthrough a plurality of cracking tubes each of which was 8 inches indiameter and 30 feet long. The cracking tubes contained a nickelcatalyst and were disposed, substantially throughout their length, in afurnace aifording external heat to the tubes. The tubes were maintainedat an increasing temperature gradient from about 1300 F. in the regionimmediately below the arch to a maximum of about 2000 F. in the regionadjacent the opposite end of the tube. The temperature of the reactantsin the zone of principal reaction was between about 1500 F. to about1900 F. The tubes were composed of type 310 stainless steel containing0.08% carbon and stabilized with 0.93% columbium. A cracking efliciencyof 96.7% was obtained and the system was substantially free of sulfurpoisoning and detrimental carbon formation.

Example 3 In the production of hydrogen, natural gas and steam in avolume proportion of 1 to 1.8 were passed through a plurality ofcracking tubes containing nickel catalyst. The tubes and furnace weresimilar to that described for Example 2. The tubes were maintained at anincreasing temperature gradient of from about 1000 F. in the regionimmediately below the arch to a maximum of about 1800 F. in the regionadjacent the opposite end of the tube. A cracking efficiency of 98 to99% was obtained in an experimental furnace operated for a test periodof over two years with no appreciable change in efficiency.

In the furnace described in the above examples, the hydrocarbons andsteam were passed downwardly and the combustion gases were passedupwardly. Thus, the high temperature combustion gases were in contactwith a very hot tube. A plurality of burners was placed at variouslevels to control the temperature of the various 7 zones as desired.

sistance to tube failure during extended operation under the hightemperature conditions illustrated in the ex-v amples. Upon examinationof such tubes after long periods of substantially continuous exposure tosuch elevated temperatures, it was surprisingly discovered that theportion of the tubes below the arch which was maintained at temperaturesbelow about 1700 F. contained very substantial amounts of sigma phase.In some instances, the sigma concentration was found to be as high as62%. Still despite the stress, elongation, distortion, and shockconditions which occur during the course of extended high temperaturecracking, no cracks or tube ruptures occurred in these regions and thecrack-free life of the entire tube was considerably greater than withthe 310 metal previously employed.

With reference to the above examples, it will be seen that theperformances obtained under prolonged periods of operation wereremarkably consistent. Although it is not alleged that the alloy used inthe present invention is a predominant factor in obtaining suchremarkable consistency in performance, there is no doubt but what it isan important contributingfactor. in control of catalytic elfect. Thiscan readily be determined by microscopic examination and comparison ofstainless steel tubes with and without stabilizing material present inthe alloy. When the stabilizing material is present in the alloy,microscopic examination at 150x reveals a uniform distribution of theingredients throughout the alloy. This condition exists even afterprolonged operation at elevated temperature. In contradistinctionthereto, when the stabilizing material is not present, microscopicexamination reveals a uniform distribution of the ingredients throughoutthe alloy initially; but after pro,- longed operation at elevatedtemperature, microscopic examination reveals a definite precipitation ofcarbides at the grain boundaries of the alloy.

This carbide precipitation causes irnpoverishmentof chromium in thealloy. For example, in comparing a 25 20-tube (25% chromium-20% nickel)after service and containing stabilizing material with a 25720 tubeafter service and without stabilizing material, in the latter tube thechromium impoverishment produced adjacent to the grain boundariesprovides a condition of chromium content below 12% over an appreciablearea as compared to 25% in other areas. The precipitated carbides mayrun as high as 90% chromium at the grain boundaries. Therefore, there isa marked nonuniformity of chromium content and a surface of nonuniformmetal analysis. Catalytically expressed, the exposed surface of the tubecontains from 90% chromium to less than 12% chromium. It will beappreciated that the transitory characteristics of unstabilizedstainless steel alloys introduce an undesirable variable in the processof cracking hydrocarbons which variable has been substantiallyeliminated in accordance with this invention.

In addition to the undesirable transitory characteristics ofunstabilized stainless steels from the catalytic standpoint, carbideprecipitation at the grain boundaries reduces the strength of the steelsand renders them unsuitable for high temperature processes.

In service, nonstabilized 25-20 stainless steel tubes, produced bydifferent methods consistently gave heavy carbide precipitation. Thisresulted in weakened crystal boundaries with eventual opening up oflarge cracks. The effect was one of shortened tube life or decreasedcapacity in order to sustain tube life.

In the catalytic steam-hydrocarbon process, carried out in a tubularfurnace, the capacity of throughput of a system is dependent upon thepressure which is allowable or practical within a tube. Pressure mayrepresent resistance to flow or it may represent in addition theresistance of subsequent equipment. In either case, the ability toWithstand increased pressure through years of operation is a much neededand distinctive improvement in the art.

' undesirable.

Furthermore, in accordance with the present invention there is provideda uniform composition of metal surface. This is desirable in catalyticprocesses where high. degree control of unwanted side reactions is anadvantage. A uniform composition of metal surface is also desirable inthermal processes such as the production of olefins and valuableby-products since the elimination of unwanted side reactions improvesthe yield and quality of product obtained.

One distinguishing characteristic between steels conducive andnonconducive to undesired reaction of hydrocarbons is magneticsusceptibility. It was found that nonstabilized type 310 steel afterlong service at temperatures above 800 F. and notably above red heatshowed substantially developed magnetic qualities. In contradistinctionthereto stabilized type 310 steel containing colum-- bium in the sameservice for similar long periods showed only the slightest trace ofmagnetic qualities. Samples suspended by thread were easily turned outof a vertical line by a magnet in the case of service exposednonstabilized type 310 steel, whereas service exposed stabilized type310 steel containing columbium could not be attracted out of line butcould be made to turn around its axis of suspension with difiiculty,there being no noticeable diiference in this case between new andservice exposed material. It is believed that this characteristic is acontributing factor to improved catalytic result in the case of hydrogenproduction and to improved thermal cracking result in the case of olefinproduction through absence or impedance of side reactions.

The advantage of the present invention are multifold. The uniformity ofsurface composition provided in ac cordance with the present inventionis a distinct improvement, particularly in processes where sidereactions are In the production of olefins from crude oils, Where duringhigh temperature cracking, conditions accelerative or conducive tocarbon formation are un- Wanted, the process of this invention has beenfound-advantageous since no appreciable decomposition to carbon takesplace. It has further been found that control of the cracking reactionhas been improved whereby ethylenepropylene weight production ratio canbe easily varied from 3 to 0.75 and even lower with less facility. It isquite normal to obtain a high ratio of ethylene to propylene, but thelow ratio of ethylene topropylene as hereinbefore set forth is difiicultto obtain. This same case of control enables other desired and valuedproducts to be obtained in maximum quantity. Furthermore, the resistanceto cracking of the thin walled tubes employed is increased despite theformation of sigma phase which has formerly been thought to acceleraterather than retard the formation of cracks. In general, the process ofthe present invention provides improvement in operability, maintenanceand longevity of a thermal or catalytic process and provides for animprovement in the quality of the products derived from such processes.

The present invention has wide applicability in proc esses such as:thermal or catalytic production of hydrogen; production of hydrogen bypartial combustion of hydrocarbons; purification of hydrogen gasescontaining NO; production of acetylene, ethylene, propylene, andbutylene; production of gas of specific B.t.u. content; production ofother liquid hydrocarbons; and in general, process use at controlledtemperature ranges from about 900 F. to highest practical temperaturesof use of the metal, at pressures below or above atmospheric withvariable quantities of steam, CO or other oxygen-containing materialfrom zero to several times the weight ratio of hydrocarbon used, andincluding a large excess of steam. The operation of such processesincluding ratio of reactants, catalysts, quality of reactants, etc., maybe carried out in accordance with normal practice with the presentinvention serving as a valuable adjunct when incorporated with suchprocesses. In addition, such processes may utilize normally gaseoushydrocarbons or normally 7 liquid hydrocarbons or mixtures thereof whileobtaining the benefits afiorded by this invention.

The process of the invention has been illustrated in the examples withtype 310 metal, modified by addition of columbium. However, the otheraustenitic alloys, such as type 309 and type 314, modified by additionof the prescribed amounts of columbium, titanium, or molybdenum, arealso operable and may be employed as desired.

The present invention is particularly applicable and advantageous in theprocesses disclosed in my US Patent Re. 21,521, and my copendingapplications: Serial No. 576,481, filed February 6, 1945, now US.2,524,840; Serial No. 678,163, filed June 20, 1946; and Serial No.692,345, filed August 22, 1946, now US. 2,525,276. Although the aboveprocesses disclose at least one externally heated elongated tube as apreferred means in conjunction with the process operation, it is evidentthat the tube or tubes may be of other configuration such as a coil, orother means constituting a suitable reaction zone, with out departingfrom the spirit and scope of the invention.

This application is a continuation-in-part of my copending applicationSerial No. 779,420, filed October 11, 1947, now abandoned, which is acontinuation-in-part of my copending application Serial No. 692,345,filed August 22, 1946, and now issued as US. 2,525,276.

What I claim and desire to protect by Letters Patent is:

1. A hydrocarbon cracking process which comprises passing a hydrocarbontogether with steam through an austenitic alloy cracking tube, applyingheat to said tube so as to maintain the temperature of same within thelimits of about 9002400 F. including a temperature gradient across atleast an upstream section thereof within the limits of about 1000 toabout 1700 F., said alloy tube comprising carbon, chromium, nickel, ironand columbium and the said ingredients being present in amounts of fromabout 0.07 to about 0.25 percent carbon, from about 20 to about 30percent chromium, from about 12 to about 25 percent nickel, from about 4to about 16 times the said carbon content of columbium and the remainder substantially iron, said tube alloy containing sigma formed inthe presence of said columbium and exhibiting thereby improvedresistance to tube failure ordinarily caused by the presence of sigmaformed within said range of about 1000 to about 1700 F.

2. A process of claim 1 wherein said hydrocarbon is finely difi'used andpassed along an inner path within said tube, said steam is passedthrough said tube concurrently with flow of said hydrocarbon and alongan outer path of said tube concentric with said inner path and incontact with the inner surface of said cracking tube, and wherein saidhydrocarbon and said steam are caused to flow into direct contact withinsaid heating tube in a downstream section thereof.

3. Apparatus for cracking hydrocarbons comprising a furnace, at leastone tube within said furnace and means for heating said tube at atemperature within the range of 900 and 2400 F., said tube being formedfrom an austenitic-type stainless steel alloy comprising carbon,chromium, nickel, iron and columbium, the said ingredients being presentin amounts, on a weight basis, from about 0.07 to about 0.25 percentcarbon, from about 20 to about 30 percent chromium, from about 12 toabout 25 percent nickel, and the remainder substantially iron; saidstainless steel containing sigma formed in the presence of saidcolumbium, whereby said tube exhibits improved resistance to failureWithin the said 1000-1700" temperature range ordinarily caused by thepresence of sigma.

References Cited in the file of this patent UNITED STATES PATENTS1,842,221 Wade Jan. 19, 1932 1,927,829 Harnsberger et al Sept. 26, 19332,081,927 Hassler et a1 June 1, 1937 2,101,835 Alcorn Dec. 14, 19372,218,495 Balcar Oct. 15, 1940 2,525,276 Shapleigh Oct. 10, 1950 OTHERREFERENCES Morton: Nickel-Bearing Alloys Used in Petroleum Refining,paper presented at 8th mid-year meeting, American Petroleum Institute,Wichita, Kansas, May 25, 1938 (10 pages).

Metals Handbook, 1939 edition, pages 535, 536 (2 pages), pub. byAmerican Society for Metals, Cleveland, Ohio.

Kinzel et al.: The Alloys of Iron and Chromium, vol. II, pages 413-429,435, 436 (19 pages), pub. by McGraw- Hill Book Co., New York (1940).

Archer: Refiner and Natural Gasoline Manufacturer, vol. 20 (July 1941),pages 66-72, (8 pages).

Transactions of American Society for Metals, vol. 39, pages 404, 437 and438 (3 pages), 1947.

Metals Handbook, 1948 edition, pages 5 63, 564 (2 pages), pub. byAmerican Society for Metals, Cleveland, Ohio.

1. AHYDROCARBON CRACKING PROCESS WHICH COMPRISES PASSING A HYDROCARBONTOGETHER WITH STEAM THROUGH AN AUSTENTIC ALLOY CRACKING TUBE, APPLYINGHEAT TO SAID TUBE SO AS TO MAINTAIN THE TEMPERATURE OF SAME WITHIN THELIMITS IOF ABOUT 900-2400* F. INCLUDING A TEMPERATURE GRADIENT ACROSS ATLEAST AN UPSTREAM SECTION THEREOF WITHIN THE LIMITS OF ABOUT 10000* TOABOUT 1700* F., SAID ALLOY TUBE COMPRISING CARBON, CHROMIUM, NICKEL,IRONAND COLUMBIUM AND THE SAID INGREDIENTS BEING PRSENT IN AMOUNTS OF FROMABOUT 0.07 TO ABOUT 0.25 PERCENT CARBON, FROM ABOUT 20 TO ABOUT 30PERCENT CHROMIUM FROM ABOUT 12 TO ABOUT 25 PERCENT NICKEL, FROM ABOUT 4TO ABOUT 16 TIMES THE SAID CARBON CONTENT OF COLUMBIUM AND REMAINDERSUBSTANTIALLY IRON, SAID TUBE ALLOY CONTAINIGN SIGMA FORMED IN THEPRESENCE OF SAID COLLUMBIUM AND EXHIBITING THEREBY IMPROVED RESISTANCETO TUBE FAULURE WITHIN NARILY CAUSED BY THE PRESENCE OF SIGMA FORMEDWITHIN SAID RANGE OF ABOUT 1000 TO ABOUT 1700* F.